I have a mower with a Briggs & Stratton SP470 engine. Mostly it runs fine but occasionally it decides to run rich and hunts, sending out puffs of black smoke. I had thought that this was due to an oil-clogged air filter and cleaned this up. This cured it, but the problem has returned, and the air filter is clean. I speculate that the cure was pure coincidence.
The problem is that there are next to no adjustments. Throttle position maybe, although that's regulated by a spring. No choke, no air control, no fine running adjustment. What can the matter be?